1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to reducing visual artifacts on bi-stable displays.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be updated electronically. Some of the desirable properties of paper that this type of display tries to achieve include: flexibility, wide viewing angle, low cost, light weight, low power consumption, high resolution, high contrast and readability indoors and outdoors. Because these displays attempt to mimic the characteristics of paper, they are referred to as Electronic Paper Displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper and bi-stable displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveals that in general, EPDs require much less power and have higher spatial resolution, but have the disadvantages of slower update rates, less accurate gray level control, and lower color resolution. Many electronic paper displays are currently only grayscale devices. Color devices are becoming available often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
Electronic Paper Displays are typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. This allows EPDs to maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously, and power is only needed when changing from one state to another. However, many EPD devices are stable at multiple states and thus support multiple gray levels without power consumption.
The low power usage of EPDs makes them especially useful for mobile devices where battery power is at a premium. Electronic books are a common application for EPDs in part because the slow update rate is similar to the time required to turn a page, and therefore is acceptable to users. EPDs have similar characteristics to paper, which also makes electronic books a common application.
While electronic paper displays have many benefits there are two problems: (1) slow update speed (also called update latency); and (2) visibility of previously displayed images, called ghosting.
The first problem is that most EPD technologies require a relatively long time to update the image as compared with conventional CRT or LCD displays. A typical LCD takes approximately 5 milliseconds to change to the correct value, supporting frame rates of up to 200 frames per second (the achievable frame rate is typically limited by the ability of the display driver electronics to modify all the pixels in the display). In contrast, many electronic paper displays, e.g. the E-Ink displays, take on the order of 300-1000 milliseconds to change a pixel value from white to black. While this update time is certainly sufficient for the page turning needed by electronic books, it is problematic for interactive applications like pen tracking, user interfaces and the display of video.
One type of EPD called a microencapsulated electrophoretic (MEP) display moves hundreds of particles through a viscous fluid to update a single pixel. The viscous fluid limits the movement of the particles when no electric field is applied and gives the EPD its property of being able to retain an image without power. This fluid also restricts the particle movement when an electric field is applied and causes the display to be very slow to update compared to other types of displays.
When displaying a video or animation, each pixel should ideally be at the desired reflectance for the duration of the video frame, i.e. until the next requested reflectance is received. However, every display exhibits some latency between the request for a particular reflectance and the time when that reflectance is achieved. If a video is running at 10 frames per second and the time required to change a pixel is 10 milliseconds, the pixel will display the correct reflectance for 90 milliseconds and the effect will be as desired. If it takes 100 milliseconds to change the pixel, it will be time to change the pixel to another reflectance just as the pixel achieves the correct reflectance of the prior frame. Finally, if it takes 200 milliseconds for the pixel to change, the pixel will never have the correct reflectance except in the circumstance where the pixel was very near the correct reflectance already, i.e. slowly changing imagery.
The second problem of some EPDs is that an old image can persist even after the display is updated to show a new image. This effect is referred to as “ghosting” because a faint impression of the previous image is still visible. The ghosting effect can be particularly distracting with text images because text from a previous image may actually be readable in the current image. A human reader faced with “ghosting” artifacts has a natural tendency to try to decode meaning making displays with ghosting very difficult to read.
FIG. 1A illustrates a ghosting artifact displayed on a bi-stable display in accordance with prior art techniques for updating a bi-stable display. The original image 102 is a large letter ‘X’ rendered in black on a white background. The next desired image is a large letter ‘O’ in black on a white background. The right side of FIG. 1A shows the image 106 after a direct update to the final value has been made, but the ‘X’ is still partially visible and appears as a faint image in the final image. The prior art systems apply the voltages to move pixels from their current state to the desired state, however, each pixel is a mix of the desired state and the original state.
FIG. 1B illustrates a prior art technique for reducing the ghosting artifacts present from normal operation as shown and described above with reference to FIG. 1A. Here, display control signals are used that do not bring each pixel to the desired final value immediately. The original image 110 is a large letter ‘X’ rendered in black on a white background. First, all the pixels are moved toward the white state as shown by the second image 112, then all the pixels are moved toward the black state as shown in a third image 114, then all the pixels are again moved toward the white state as shown in the fourth image 116, and finally all the pixels are moved toward their values for the next desired image as shown in the resulting image 118. Here, the next desired image is a large letter ‘O’ in black on a white background. Because of all the intermediate steps this process takes much longer than the direct update. However, moving the pixels toward white and black states tends to remove some of the ghosting artifacts as can be seen by comparing the prior art output image 106 with the result image 118. The residual artifact “X” in FIG. 1B is less visible than the artifact shown in FIG. 1A, but is still present.
Setting pixels to white or black values helps to align the optical state because all pixels will tend to saturate at the same point regardless of the initial state. Some prior art ghost reduction methods drive the pixels with more power than should be required in theory to reach the black state or white state. The extra power insures that regardless of the previous state a fully saturated state is obtained. In some cases, long term frequent over-saturation of the pixels may lead to some change in the physical media, which may make it less controllable.
One of the reasons that the prior art ghosting reduction techniques are objectionable is that the artifacts in the current image are meaningful portions of a previous image. This is especially problematic when the content of both the desired and current image is text. In this case, letters or words from a previous image are especially noticeable in the blank areas of the current image. For a human reader, there is a natural tendency to try to read this ghosted text, and this interferes with the comprehension of the current image. Prior art ghosting reduction techniques attempt to reduce these artifacts by minimizing the difference between two pixels that are supposed to have the same value in the final image.
It would therefore be highly desirable to produce an electronic paper display that requires a relatively short time to update a displayed image and displays less “ghosting” artifacts when a new image is updated on the display screen.